Fluorescent pteridine nucleoside analogs that can be substituted for a native base within the DNA have been developed and patented. The fluorescence properties of these probes are being used very successfully in the study of protein/DNA interactions. Because they are base-stacked and base-paired in the DNA, they reveal features of DNA structure that cannot be seen with standard linker-attached probes which are usually too far away from the bases to reveal these subtleties. The understanding of the fine structure of DNA is a key to many life processes. The probes provide us with a new tool with which to measure these subtle associations. Myers et al (2003) have used a pteridine analog to study structure/function relationships within tetrad formations. Sanabia et al and Stanley et al have demonstrated the usefulness of the probes in single molecule studies.